RESUMEN
NIS Synthetases are a widely distributed, novel superfamily of enzymes critical to stealth siderophore production-small molecules increasingly associated with virulence. Study of these enzymes for inhibition or utilization in biosynthesis of new antibiotics has been hindered by multiple kinetics assays utilizing different limiting reporters or relying on product dissociation as a precursor to signal. We present a label free, continuous readout assay optimized for NIS Synthetase systems utilizing an isothermal titration calorimetry instrument. This assay has been tested in an iterative system comparing multiple turnovers on a single substrate to a single bond formation event and is able to delineate these complex kinetics well. The ITC-based kinetic assay is the first label-free assay for the NIS field, which may allow for more detailed kinetic comparisons in the future, and may also have broader use for iterative enzymes in general.